1. Field of the Invention
The present invention relates to a wavelength selective switch that can demultiplex or multiplex lights of different wavelengths in optical wavelength division multiplexing transmission.
2. Description of the Related Art
As the optical wavelength division multiplexing transmission becomes widespread, the wavelength selective switch for multiplexing or demultiplexing a light signal depending on wavelengths has become a key device of the optical transmission.
Here, FIG. 1 shows a schematic block diagram of a prior-art wavelength selective switch. Input/output ports 101 refer to all input/output ports (input/output ports 101a to 101e in FIGS. 1 and 2) at an input/output terminal 100. Waveguides 14 refer to all waveguides (waveguides 14a to 14e in FIGS. 1 and 2) in a fiber array 140.
The wavelength selective switch 200 in FIG. 1 includes a lens array 102 for converting lights into parallel lights, the lights output from the input/output ports 101 of the input/output terminal 100 disposed at a focal position, a first lens 103 with a high numerical aperture and for converging the lights from the lens array 102, a second lens 104 disposed to have a focal position in common with the first lens 103, a spectral element 105 for reflecting the lights from the second lens 104 at different angles for respective wavelengths, and a mirror array 106 disposed at the focal position of the second lens 104 to reflect the lights that have passed through the second lens 104 from the spectral element 105 at arbitrary angles toward the second lens 104 (e.g., see Japanese Laid-Open Patent Publication NO. 2003-101479; Japanese Laid-Open Patent Publication No. 2006-276216; and Japanese Laid-Open Patent Publication No. 2006-284740).
With this, the lights reflected by the mirror array 106 at the arbitrary angles can be converged on different input/output ports for different wavelengths according to angles of individual micromirrors of the mirror array 106. In this way, the first lens 103 has functions of changing angles of the lights that have been reflected by the spectral element 105 and passed through the second lens 104 out of the lights reflected by the mirror array 106 and giving offsets to optical axes of the lights from the input/output ports 101.
FIG. 2 shows a schematic block diagram of a wavelength selective switch 200′ in which the reflective spectral element 105 in FIG. 1 is replaced with a transmissive spectral element 105′. Parts having the same functions as those of the wavelength selective switch 200 in FIG. 1 are provided with the same reference numerals. A third lens 104′ in FIG. 2 has the same functions performed for the lights from the spectral element 105′ to a mirror array 106 as the second lens 104 in FIG. 1.
In a case of an N-input and 1-output (Add-type) wavelength selective optical switch, one of input/output ports of the wavelength selective switch 200′ can be used as an output port and the others can be used as input ports in FIG. 2. In the following description, they will be referred to as an output port 101c, an input port 101a, an input port 101b, an input port 101d, and an input port 101e. 
Wavelength-multiplexed light signals are emitted as diverging lights from the input ports 101 through the waveguides 14 in the fiber array 140. For example, the light signal (two-dot chain line) emitted as the diverging light from the input port 101b enters a lens array 102 to be converted into a parallel light and enters a first lens 103. The light signal that enters the first lens 103 is converted into a converging light, forms an image at a point A, turns into the diverging light again, enters the second lens 104 to be converted into a parallel light again, and enters the spectral element 105. The light signal that enters the spectral element 105 is demultiplexed into respective wavelengths, enters the third lens 104′ to be converted into a converging light, and forms images for respective wavelengths at the mirror array 106 as shown in FIG. 3. For example, a micromirror 106c in FIG. 3 is inclined at an angle α that is necessary to cause an incident light signal λ3 to enter the output port 101c and the light signal λ3 enters the third lens 104′ as a diverging light shown by solid lines in FIG. 2. The reflected light signal that has entered the third lens 104′ is converted into a parallel light, passes through the spectral element 105, enters the second lens 104 to be converted into a converging light, and forms an image at the point A. The reflected light signal that has formed the image at the point A turns into a diverging light, enters the first lens 103 to be converted into a parallel light, enters the lens array 102 to be converted into a converging light, is coupled to the output port 101c, and is transmitted through the fiber array 101.
Here, if a wavelength-multiplexed light signal is also emitted simultaneously from the input port 101a, it forms a light path as shown by a one-dot chain line in FIG. 2, is demultiplexed by the spectral element 105, and forms an image at the mirror array 106 as in FIG. 2. For example, because the micromirror 106c is already inclined at the angle α, the light signal λ3 that has formed the image on the micromirror 106c is reflected in a light path shown in a broken line in FIG. 2 and enters not the output port 101c but a vicinity of the input port 101a. To cope with this, an in-line isolator 109 is inserted into midway of fibers of the fiber array corresponding to the input ports to thereby prevent the light signals from propagating in an opposite direction through the fibers.